Gen drive light will illuminate if IDG low oil pressure caused by : IDG failure,engine shutdown ,IDG automatic disconnect due to high oil temp,and IDG disconnected through generator drive DISCONNECT switch. So this light will be ON.
SOURCE OFF light : illuminated when :' no source was manually selected to power the related transfer bus ,OR the MANUALLY selected source has been disconected.' In your case this light will be ON (as FD said,the last manually selected source was disconnected)
Brgds
Alex